1. Field of the Invention
The present invention relates to spark plugs for use in internal combustion engines of motor vehicles.
2. Description of the Related Art
Internal combustion engines mounted to motor vehicles use various types of spark plugs in order to ignite a fuel in a combustion chamber. For example, a spark plug has a conventional structure in which a spark discharging gap is formed between a center electrode and an earth electrode. Spark discharge is generated in the spark discharging gap formed between the center electrode and the earth electrode in order to ignite a mixture gas composed of air and a fuel in the combustion chamber of the internal combustion engine. There is another type of a spark plug having a structure in which an electrode chip is formed on the center electrode or the earth electrode in order to increase an ignition capability, etc.
Recently, there is a demand for improving a wear resistance of an electrode chip used in a spark plug in view of a temperature increase in a combustion chamber of an internal combustion engine because the internal combustion engine has a high performance, etc. There are spark abrasion or spark wear and oxidation abrasion or oxidation wear which abrades an electrode chip in a spark plug. In the spark abrasion, a surface of the electrode chip is instantaneously melted by the spark discharge. On the other hand, in an occurrence of oxidation abrasion, a surface of an electrode is oxidized and vapored when the spark plug is used in a high temperature environment. For example, a Japanese patent laid open publication No. JP H09-298083 has disclosed a spark plug having a conventional structure in which an electrode chip is made of iridium Ir having a high melting point and a superior spark abrasion resistance capability. In addition to iridium, the electrode chip contains platinum Pt and rhodium Rh having a superior oxidation resistance.
However, because the conventional electrode chip used in the spark plug disclosed in JP H09-298083 is made of noble metals such as iridium, platinum and rhodium, this increases a manufacturing cost of the electrode chip and the spark plug. So, there is a demand for providing a spark plug having a superior spark discharging wear resistance capability, a superior oxidation resistance and a long life with a low manufacturing cost.